This invention relates to a color image forming apparatus such as a tandem-type color printer or copying machine and a complex machine of these having an intermediate transfer belt or a photoreceptor belt and an image forming method suitably applied thereto.
In recent years, a tandem-type color printer or copying machine or a complex machine of these has been used in many cases. In these color image forming apparatus, there are provided exposure means, developing devices, and photoreceptor drums for the respective colors of yellow (Y), magenta (M), cyan (C), and black (BK), an intermediate transfer belt,. and a fixing device.
For example, in the exposure means for the color Y, an electrostatic latent image is formed on the photoreceptor drum on the basis of arbitrary image information. In the developing device, a color image is formed through the deposition of toner particles of the color Y on the electrostatic latent image formed on the photoreceptor. The toner image is transferred from the photoreceptor drum onto the intermediate transfer belt. As regards the other colors M, C, and BK, the same processing is carried out. The color toner image, having been transferred onto the intermediate transfer belt, is fixed by the fixing device.
Incidentally, by a color image forming apparatus of this kind, a color toner image has to be formed on an intermediate transfer belt without color deviations (positional deviations of component color images from one another, that is, component color images being out of registration). The reason is that the color toner image composed of component color images superposed without color deviations should be transferred onto a paper sheet.
FIG. 54 is a drawing showing an example of the detection of color registration marks according to a prior art. In FIG. 54, a processing called “color registration mark detection” is applied to the intermediate transfer belt 6 (also called the belt unit or paper conveyance belt) periodically or non-periodically before a color image is formed on the basis of arbitrary image information. In this detection processing, by the use of reflection-type photo-sensors (hereinafter also referred to as registration sensors) 12A or 12B, etc., the “7-shaped” color registration marks (marks made up of a horizontal straight line segment and an oblique straight line segment meeting together at a point with an acute angle, and hereinafter referred to also as color registration marks CR simply) are detected.
At this time, the light, having emitted from the photo-sensors 12A or the like, is intercepted by the color registration mark CR, for example, on the intermediate transfer belt 6. In this processing, the detection of the mark position (the edge or the center of gravity) of the registration mark CR is done in such a way that the light traveling after being reflected by the intermediate transfer belt 6 is detected. The edge detection data are recorded in a RAM or the like, and afterwards, on the basis of the record, the amounts of positional deviations for the component colors Y, M, C, and BK are calculated; the exposure timing at the exposure means for the respective colors are controlled in such a way as to superpose the toner images with these amounts of positional deviations reduced to zero.
FIG. 55 is a drawing showing an example of the waveform of a signal of a scratch obtained by the registration sensor 12A or the like. In FIG. 55, the abscissa represents time t, and the ordinate represents the signal level of a position detection signal S2 obtained by the registration sensor 12A or the like. The solid line in FIG. 55 shows the waveform of a signal representing the state of the color image formation surface of the intermediate transfer belt 6 before the formation of the color registration marks. Lb denotes the base correction level of the position detection signal S2, and Lth denotes a threshold voltage.
This waveform can be obtained by the detection of the color image formation surface by means of the registration sensor 12A or the like through one revolution of the intermediate transfer belt 6. Whether or not the interception of light by the color registration marks does occur in the detection processing is judged by whether or not a signal level lower than the threshold voltage Lth does exist. According to the signal example shown in FIG. 55, values of the position detection signal S2 coming lower than the threshold voltage Lth are detected; this is the case where the intermediate transfer belt 6 has scratches etc. to obstruct the color registration mark detection processing. It sometimes occurs that scratches are produced when the intermediate transfer belt 6 is rubbed by the drum 1 which is suddenly stopped due to a power failure during the operation of the image forming apparatus, or when the intermediate transfer belt 6 is taken out and inserted for maintenance operation.
Incidentally, according to a tandem-type color image forming apparatus based on a conventional method, there are problems as described below.
(1) In the color registration mark detection processing, owing to the change of the intermediate transfer belt 6 with the passage of time, for example, scratches on the belt surface, although they are different depending on the material of the belt, the reflected light quantity by the color toner layer is raised, which makes it difficult to secure sufficiently the signal difference between the toner surface and the substrate surface. In particular, if the image density of the color registration marks CR fluctuates, that is, if the density becomes lower, or if the density becomes too much higher, the reflected light quantity becomes higher; therefore, there is a possible risk that the color registration marks cannot be detected with a good reproducibility due to the density fluctuation.
(2) If scratches (permanent scratches) increase on the belt owing to the above-mentioned change of the intermediate transfer belt 6 with the passage of time, it occurs frequently the case where the sensors react to the scratches etc. on the belt, which makes it impossible to detect exactly the mark edge of the color registration marks (hereinafter referred to also as the mark images). In this connection, it can be thought of a method in which a black colored band is formed on the color image formation surface of the intermediate transfer belt 6 beforehand, and afterwards, registration marks of the respective colors are formed on this black surface, but this makes toner consumption larger.
(3) If scratches (permanent scratches) increase on the belt owing to the change of the intermediate transfer belt (hereinafter referred to also as the image transfer system) with the passage of time, in the color registration mark detection processing, it occurs frequently the case where the sensors react to the scratches etc. on the belt, which makes it impossible to detect exactly the mark edge of the color registration marks. In such a case, it is necessary to take trouble to form the color registration marks at positions of no belt scratches.
(4) In this connection, for a method of eliminating scratch data from position detection data containing belt scratches, it has been thought of a method in which registration marks are formed, afterwards, the periods of time from a reference timing to the line edges of each of the registration marks are memorized, this processing is repeated plural times, and the detection of all the registration marks is tested; then, only the places where the registration marks overlap one another are extracted. However, as regards this method, in order to eliminate. belt scratches etc. which appear unexpectedly, the above-mentioned processing has to be repeated plural times every occasion, it takes a considerable time for the calculation of the color deviation amounts of one time.